This invention is directed to improvements in the manufacture of fiberboard shims for use in the assembly of automobiles.
Shims function by filling gaps between automotive components. The gaps, or spaces, develop as a result of improperly aligned or improperly sized parts, and often cause undesirable audible vibrations in the automobile during operation and occasionally cause mechanical failure.
Such shims produced from steel are known. These shims are produced in a variety of shapes and thicknesses. Steel shims are dimensionally stable as a result of the physical properties of the steel itself. However, shims made from steel are expensive to produce and tend to rattle after being driven into place, resulting in undesirable audible vibrations.
Subsequently, it was found that shims could be produced from cellulosic materials which were hardened by means of a curable impregnated resin. Typically, the raw cellulosic material was processed to form fiberboard of a specific thickness, which was then cut to the desired shape. The cut shims were then impregnated with a hardenable material, such as a phenolic resin. Shims produced in this way were cheaper than steel shims and did not audibly rattle once in position. However, the fiberboard shims tended to deform and delaminate while being tapped into place, and further tended to shrink and absorb moisture during use.
The operational characteristics of fiberboard shims were improved by immersing the fiberboard shims in a solvent-based solution of methylene-bis-(4-phenyl isocyanate) as described in Decker et al U.S. Pat. No. 4,029,846. The named isocyanate, which was critical to the improvement, was believed to react with the terminal ends of the cellulose to produce a chemical- and moisture-resistant shim. Shims made according to the Decker patent resisted compression, exhibited good torque retention, and resisted deformation and delamination while being tapped into place.
Production of fiberboard shims according to the teachings of the Decker '846 patent has certain significant disadvantages. First, the immersion of the shims into the isocyanate solution causes an uneven distribution of isocyanate into the shim body. This limits the thickness of the shim. Further the thicker the shim the weaker it is since only the surface is impregnated. Further the curing temperature causes degradation of the untreated portions.
Also, the immersion step requires additional equipment, additional production time, and special solvents which are considered to be a potential health hazard. Handling isocyanate impregnated material is also quite dangerous.
The Decker patent states that a phenol blocked isocyanate is unsuitable which is accurate relative to the Decker method. Specifically, the required cure temperature degraded the fibers. Further, it was difficult to get the blocked isocyanate into solution.